Micropillar compression tests of Ti5Si3 single crystals were conducted at room temperature as a function of loading axis orientation and specimen size in order to investigate their room temperature plastic deformation behavior. Plastic flow by the operation of three deformation modes, {1100}[0001], {2112} < 2113 > and {1101} < 2113 > slip were observed in [2205]-, [0001]- and [4310]-oriented micropillar specimens deformed at room temperature, respectively. The CRSS values were evaluated to be very high above 2.7 GPa and were confirmed to increase up to about 6 GPa with the decrease in the specimen size. The fracture toughness values are evaluated to be 0.45 MPa m^1/2 (notch plane // (0001)) and 0.73 MPa m^1/2 (notch plane //(1100)) based on the results of micro-cantilever bend tests of chevron-notched specimens. The fracture toughness values are considerably lower than those for D8l-Mo5SiB3  and D8l-Nb5Si3 evaluated by the same method, indicating the inherent brittleness of binary Ti5Si3 compared to the other transition-metal silicides of the TM5Si3 type (TM: transition-metal).